Stokes parameters of resonance lines scattered by a moving, magnetic medium

Theory of the two-level atom

¹ Institut d'Astrophysique Spatiale, Université Paris XI, 91405 Orsay, France
² Département Atomes et Molécules en Astrophysique, Observatoire de Paris-Meudon, 92195 Meudon, France

Received: 17 July 2001
Accepted: 14 December 2001

Abstract

The aim of the present work is to present theoretical results on the Stokes parameters of a resonance spectral line, scattered by moving atoms (or ions) in the presence of a local magnetic field. We assume that the scattered line is sensitive to the Hanle effect due to the magnetic field and also to Doppler redistribution due to the atomic motions. The present theory is developed for a two-level atom, in the framework of the density matrix formalism Blum (1981). Analogous results given in Sahal-Bréchot et al. (1986) for the magnetic-field effect alone, and in Sahal-Bréchot et al. ([CITE]) for the velocity-field effect alone, can be obtained from our theory by cancelling in the equations, respectively, the velocity field or the magnetic field. The results of our theory are general and can be used for astrophysical studies concerning the Hanle effect and the Doppler redistribution effect on the linear polarization parameters of the scattered radiation. They can be used particularly to interpret linear polarization of coronal spectral lines to get a complete determination of vectorial quantities such as the coronal magnetic field and the solar wind velocity field vectors. As an application, the atomic velocity field distribution is supposed to be Maxwellian with a drift velocity field vector. This latter describes the macroscopic motion of the scattering atoms. In the solar corona, it can be assimilated into the solar wind velocity field vector.